1. Field of Invention
The invention relates to the field of actuation systems for positioning a control surface mounted on an aircraft airfoil, and in particular, to aircraft having swept wings and externally mounted actuators.
2. Description of Prior Art
In conventional commercial and military transports, and general aviation type aircraft, airfoil cross-sections are relatively thick and provide ample room to mount actuators for the control surfaces within the airfoil envelope. But with modern high-speed, and in particular, supersonic fighter type aircraft, airfoils are relatively thin. Locating the actuator within this type of airfoil tends to occupy too much space and complicates airfoil structural design. Advanced materials, such as graphite expoxy composites, permit aerodynamic improvements by allowing the use of even thinner airfoils which would make it impossible to incorporate the actuators within the airfoil structure. This, in some instances, has led to unusual placement of the actuator system within the airfoil. For example, U.S. Pat. No. 2,984,436, "Control Means For Airfoils," by J. B. Murray, et al. Murray, et al; places the hydraulic actuator substantially parallel to the axis of rotation of the control surface (in a spanwise direction). Each end of the actuator is coupled to one end of a bellcrank, mounted at its center to the airfoil structure. A link is pivotally attached between the opposite end of each bellcrank and to the control surface. Thus, spanwise extension and retraction of the actuator is converted into longitudinal movement of the link, which rotates the control surface about its hinge axis. While this design is compact, space within the airfoil is still occupied by the actuator, reducing the fuel storage capacity of the airfoil. This actuator system can also complicate the structural design of the airfoil. Other examples of control surface actuation systems using internally mounted actuators are disclosed in U.S. Pat. Nos. 4,053,124, "Variable Camber Airfoil," by J. B. Cole and 4,131,253, "Variable Camber Trailing Edge For Airfoil," by E. J. Zapel.
To avoid these problems it is quite common to externally mount the hydraulic cylinders as disclosed in U.S. Pat. No. 4,181,275, "Apparatus For Operating Flap Means Secured To the Wings of An Aircraft", by G. Moelter et al. The Moelter, et al., system mounts the actuator below the airfoil and controls the position of the control surface through a sophisticated linkage system. The problem here is, if the wing is swept the movement of the point of connection of the actuator to the linkage coupled to the surface moves out of the plane of the direction of actuation as the control surface rotates. Thus, one end of the actuator must be capable of translating in a spanwise direction. This, requires the actuator fairing (necessary for drag reduction) be larger than would be needed if the actuator did not have to translate in the spanwise direction. That portion of the fairing which moves with the control surface does not move in the streamwise plane of the fixed fairing, causing transverse displacement between the two, and additional drag. Of course the larger fairing results in greater drag. Another example of an externally mounted hydraulic actuator can be found in U.S. Pat. No. 2,395,671, "Control Means For Airfoils," by E. S. Kleinhans, et al.
It is therefore a primary object of this invention to provide an actuator system for positioning a control surface mounted on a swept airfoil of an aircraft which compensates for the relative motion of the control surface relative to the longitudinal axis of the aircraft.
It is another object of this invention to provide an actuation system having an externally mounted actuator for positioning a control surface mounted on a swept airfoil of an aircraft which minimizes the size of the fairing necessary to enclose the actuator.